1. Field of the Invention
The present invention relates to a phase-change optical recording medium in which a reversible phase-change between the crystalline and amorphous phases is brought about by light beam irradiation so as to record information.
2. Description of the Related Art
(Principle of Phase-Change Optical Recording Medium)
The phase-change optical recording medium, which uses a phase-change optical recording film that permits bringing about a reversible phase-change between the crystalline and amorphous phases upon irradiation with a light beam, operates according to the following principle. That is, recording is performed by heating the region irradiated with a light beam to temperatures higher than the melting point of the film so as to melt the irradiated region, followed by rapidly cooling the molten region so as to change atomic arrangement in the cooled region in a manner to form an amorphous phase. Erasure is performed by maintaining for at least a prescribed time the temperature of the region irradiated with the light beam such that the temperature falls within a range between a level not higher than the melting point and a level not lower than the crystallizing temperature. In this stage, in the case where the initial state is crystalline, the crystalline phase is maintained, and in the case where the initial state is amorphous, the amorphous phase is changed into the crystalline phase. Reading is performed by converting the intensity of the reflected light into the intensity of an electrical signal by utilizing the phenomenon that the intensity of the reflected light from the amorphous region differs from that of the reflected light from the crystalline region, followed by subjecting the electrical signal to analog-to-digital conversion.
It should noted that the recording-reading can be performed by utilizing phase-change between a metastable crystalline phase and a stable crystalline phase as in martensite or phase-change among metastable crystalline phases as well as the phase-change between the crystalline phase and the amorphous phase noted above.
(Methods for Increasing Recording Density)
The amount of information that can be recorded on a single recording medium, i.e., the recording capacity, is considered to be increased by the two methods described below.
A first method is to shorten the pitch of the recording marks in the track direction. In this method, however, when the pitch between recording marks is significantly reduced, the pitch will be smaller than the size of a read beam. In such a case, two recording marks may be included temporarily in a read beam spot. Where the recording marks are sufficiently separated from each other, the read signal is greatly modulated so as to make it possible to obtain a signal having high amplitude. However, where the recording marks are close to each other, the obtained signal has low amplitude, with the result that errors tend to occur in the stage of converting the obtained signal into digital data.
Another method for improving the recording density is to shorten the track pitch. This method makes it possible to increase the recording density without being affected significantly by decrease in the signal intensity that is caused by the mark pitch reduction noted above. In this method, however, a so-called cross-erase problem is generated. Specifically, if the track pitch is made substantially equal to or smaller than a light beam size, the information in a certain track deteriorates when writing or erasure is performed on the adjacent track.
The cross-erase problem is generated partly because the mark on the target track is irradiated directly with the edge of the laser beam on the adjacent track, and partly because heat generated in the recording stage flows into the adjacent track so as to raise the temperature of the mark on the adjacent track and, thus, to deform the mark. In order to increase the recording density of the phase-change optical recording medium, it is necessary to overcome the problems pointed out above. Also, in order to suppress the probability of read errors for small recording marks to a low level, it is desirable that the recording marks be formed in a manner to have an even contour so as to suppress noise components as much as possible.
(Increase in Recording Capacity by Using Dual-Layer Disc)
Another method for increasing the recording capacity is to stack a plurality of information layers each including a phase-change optical recording film (see Japanese Patent Disclosure (Kokai) No. 2000-322770). The recording medium that is designed such that two information layers are stacked one upon the other and the information is written to and read from one side is called a single-sided, dual-layer disc or is simply called a dual-layer disc. It is possible to stack two single-sided, dual-layer discs so as to form a double-sided, quadruple-layer disc for further increasing the recording capacity. In the single-sided, dual-layer disc, it is necessary for the information layer close to the light incident side, which is hereinafter called L0, to have at least about 50% of transmittance. This is because it is important to prevent the light from being attenuated excessively in the information layer L0 close to the light incident side in accessing to the information layer remote from the light incident side, which is hereinafter referred to as L1. Such being the case, it is necessary for the phase-change optical recording film included in the L0 information layer to be very thin, i.e., not thicker than 10 nm. In the case of such a thin recording film, the retention time required for the crystallization is prolonged, with the result that the recording mark fails to be erased completely at the normal recording speed (reduction of erasure rate).
As a measure against the difficulty, it is known that it is effective to substitute Sn for a part of the GeSbTe recording film (Proceedings of The 12th Symposium on Phase-Change Optical Information Storage PCOS 2000, pp. 36–41). Also, it is known that it is effective to substitute Bi, In, Sn and Pb for a part of the GeSbTe recording film (see Japanese Patent Disclosure No. 2001-232941). However, in order to compensate for the crystallizing speed that is reduced in accordance with the decrease in the thickness of the recording film, it is insufficient to simply take a measure such as a change in the composition of the recording film material. Such being the case, it has been proposed to arrange an interface film effective for promoting the crystallization at the interface with the recording film. For example, it has been proposed to arrange a germanium nitride (GeN) film, a silicon carbide (SiC) film or a silicon nitride (SiNx) film as the interface film (see “Proceeding” referred to above). However, as a result of extensive research conducted by the present inventors, it has been found that cross-erase is generated in the combination of the thin recording film having a thickness of 5 to 7 nm and the conventional interface film such as GeN and, thus, it is difficult to sufficiently shorten the track pitch. It has also been found that any of the interface films referred to above has a high extinction coefficient in the wavelength of the laser light (405 nm) used in the next-generation high-density optical disc, incurring a very high optical loss.
In contrast, the recording medium in which the interface film is not arranged permits suppressing the recrystallization of the molten portion so as to suppress the cross-erase to a low level, but has an insufficient erasure rate. Also, in the L1 information layer, it is necessary to perform recording and erasure by laser light the intensity of which has been halved as a result of the passage through the L0 information layer. It follows that an increase in sensitivity of the recording medium is required.
(Methods for High-Speed Recording)
High-speed recording is also required for the phase-change optical recording medium. For example, if the recording can be performed in a time shorter than the actual viewing time, it is possible to easily realize the so-called “time-shift function” that the previous images can be viewed in the copying stage of the distributed recording medium or during the real-time recording of the broadcasting images. However, one of the factors for inhibiting the high-speed recording in the phase-change optical recording medium is a problem that the information fails to be erased completely when crystallization is performed in the overwriting stage by a laser beam at a relatively low erasure level, i.e., the problem of the insufficient erasure rate. To be more specific, since the recording mark passes through the laser spot at a high speed, it is difficult to maintain the recording mark for a sufficiently long time in temperature range within which the crystallization can be performed, with the result that the information fails to be erased completely.
Japanese Patent Disclosure No. 11-213446 discloses that an interface film formed of a material such as GeN is arranged in contact with the recording film so as to promote the crystallization and, thus, to increase the erasure rate. However, as a result of experiments conducted by the present inventors in which the material disclosed in the above Disclosure was used as the interface film, it has been found that the portion molten in the recording stage is partially recrystallized, and that it is necessary to melt a region larger than the recording mark in order to form a recording mark of a required size. It should be noted that the use of the interface film noted above requires makes it necessary to melt the region larger than required, resulting in promotion of the cross-erase, which brings about an adverse effect in terms of high-density recording. In other words, if the recording is performed using a laser power within an acceptable range in terms of the cross-erase, the width of the formed recording mark is made small, which brings a lowered carrier-to-noise ratio (CNR). On the other hand, the recording medium having no interface film permits suppressing the recrystallization in the molten portion so as to suppress the cross-erase to a low level, but the medium has an insufficient erasure rate. Therefore, it is desirable to develop a novel interface film material that permits suppressing the recrystallization in the molten portion in the recording stage while promoting the crystallization rate in the erasure stage.
(Interface Film Material)
A material containing a mixture of Ta2O5 and SiC as main components, which is a material for a sulfur-free protective film, is known as a material that can be used for an interface film capable of promoting crystallization (see Japanese Patent Disclosure No. 2003-67974). This prior art of Japanese Patent Disclosure No. 2003-67974 is mainly intended to improve the DVD using the wavelength λ of 650 nm of the laser diode (LD) that is widely used nowadays. However, the material disclosed in the Disclosure is opaque to the wavelength λ of 405 nm of the next-generation blue LD, leading to a higher optical loss. Therefore, problems remain unsolved when the particular interface film is used in the next-generation recording medium having a high recording density. Thus, an interface material which is optically transparent to the wavelength λ of 405 nm and capable of promoting the crystallization, has not yet been developed.
(Film Design of Phase-Change Optical Recording Medium)
In the phase-change optical recording medium, an amorphous mark or data is written in a desired portion of the recording film by irradiating the desired portion of the recording film with a pulsed laser beam. In contrast, the recording film is irradiated with a laser beam so as to crystallize the amorphous mark, thereby erasing the data. In the former stage, an amorphous mark is formed by rapidly cooling the laser beam-irradiated portion, and in the latter stage, the amorphous portion is crystallized by gradually cooling the laser beam-irradiated portion. Also, the recording and erasure can be performed with a lower laser power when the absorbance in the recording film is high. In contrast, a higher laser power is required for the recording and erasure when the absorbance in the recording film is low. It should be noted that the absorbance in the recording film is determined by the optical characteristics of the recording medium formed of a multi-layered film. What is also important is the thermal design relating to the film structure as to, for example, whether a rapid cooling structure is employed, even if the absorbance is the same. Thus, in the film design of the phase-change optical recording medium, the optical design and the thermal design are mainly taken into consideration. For the optical design, the optical characteristics of each thin film are required. Also, for the thermal design, the thermal properties including the melting point of each thin film, the latent heat of melting, and the crystallization temperature are required. It should be noted in this connection that it has been clarified by some researches that the thermal properties of a thin film in a nanometer order differ from those of the bulk. However, the thermal properties of the thin film could not be measured while eliminating the effects of the other factors. Therefore, empirical parameters for correcting the above thermal properties are required.